This invention relates to the field of optical fiber amplifiers and, more particularly, to means of applying pump energy to an optical fiber amplifier.
As is known in the art, an optical amplifier is a device that increases the amplitude of an input optical signal fed thereto. If the optical signal at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. A conventional fiber amplifier comprises a gain medium, such as a single mode glass fiber having a core doped with a rare earth material, connected to a WDM coupler which provides low insertion loss at both the input signal and pump wavelengths. The input signal is provided, via the coupler, to the medium. Excitation occurs through optical pumping from the pumping source. Pump energy that is within the absorption band of the rare earth dopant is combined with the optical input signal within the coupler, and applied to the medium. The pump energy is absorbed by the gain medium, and the input signal is amplified by stimulated emission from the gain medium.
Such amplifiers are typically used in a variety of applications including, but not limited to, amplification of weak optical pulses such as those that have traveled through a long length of optical fiber in communication systems. Optical amplification can take place in a variety of materials including those materials, such as silica, from which optical fibers are formed. Thus, a signal propagating on a silica-based optical fiber can be introduced to a silica-based optical fiber amplifier, and amplified by coupling pump energy into the amplifier gain medium.
Fiber amplifiers are generally constructed by adding impurities to (i.e. xe2x80x9cdopingxe2x80x9d) an optical fiber. For a silica-based fiber, such dopants include the elements erbium and ytterbium. For example, one type of fiber amplifier referred to as an erbium (Er) amplifier typically includes a silica fiber having a single-mode core doped with erbium ions (conventionally denoted as Er3+). It is well known that an erbium optical fiber amplifier operating in its standard so-called three level mode is capable, when pumped at a wavelength of 980 nanometers (nm), of amplifying optical signals having a wavelength of approximately 1550 nanometers (nm). Likewise, an amplifier having a silica-based fiber xe2x80x9cco-dopedxe2x80x9d with erbium and ytterbium shows excellent amplification of a 1550 nm optical signal when pumped with a wavelength of approximately 1060 nm. Since 1550 nm is the lowest loss wavelength of conventional single-mode glass fibers, these amplifiers are well-suited for inclusion in fiber systems that propagate optical signals in the wavelength vicinity of 1550 nm.
It has been an ongoing pursuit in the field of optical fiber amplifiers to increase the power output of the amplifiers. Traditionally, pump energy is applied to the gain medium by coupling into the doped fiber either in the same propagation direction as the signal to be amplified (referred to as xe2x80x9cco-pumpingxe2x80x9d), or by coupling it into the doped fiber in the opposite direction as the signal to be amplified (referred to as xe2x80x9ccounter-pumping). Each of these pumping methods has its own advantages, but also its own limitations. It is an object of this invention to go beyond these traditional pumping methods to provide a high power optical amplifier by providing a new means of pumping a doped optical fiber.
In accordance with the present invention, an optical fiber amplifier is provided in which a doped optical fiber gain medium is pumped by pump energy that is oscillated through a substantial portion of the gain medium. That is, a resonant cavity for the pump energy is formed that includes the amplifier fiber, such that the pump energy is reflected back and forth through the gain medium. The output coupling for the resonant cavity is absorption by the doped fiber, which results in amplification of the optical signal by stimulated emission as it passes through the fiber.
The optical pumping apparatus used to generate the oscillating pump energy may take a number of different forms. In general, reflectors that reflect optical energy at the pump wavelength are coupled to either side of the optical fiber, and reflect the pump energy back and forth through the gain medium. In one embodiment, each reflector is located in its own optical pathway separate from the optical fiber, a first of these pathways being coupled to a first side of the optical fiber while the second is coupled to the second side of the fiber. The coupling is preferably by wavelength selective couplers, such as WDMs, so that only the pump energy is diverted from the signal path and directed to the reflectors. In one variation of this embodiment, each reflector is coupled to a pump energy generator, preferably in the form of a pumped optical fiber, so that pump energy is generated on either side of the optical fiber. In another variation, the pump energy is generated at only one side, while the other side has only a reflector. In either case, the pump energy is oscillated in the pathway between the reflectors, providing the desired oscillation of pump energy through the fiber gain medium.
When the pump energy is coupled into the optical fiber using wavelength selective couplers, another variation of the invention involves using a plurality of pump wavelengths, each of which is within the absorption band of the doped optical fiber. In such an embodiment, a plurality of reflectors may be used in each of the two pump energy pathways located, respectively, to either side of the optical fiber. The different pump wavelengths are preferably close in wavelength, and each set of reflectors (i.e. each group of reflectors located to one optical side of the doped fiber) may be coupled together using narrowband wavelength selective couplers, such as narrowband WDMs. Furthermore, some or all of the reflectors may be coupled to pump sources that generate optical energy at the desired pump wavelengths.
In each of the above embodiments, the optical fiber may be doped with erbium/ytterbium (Er/Yb), which provides amplification of a 1550 nm optical signal when the fiber is pumped at a wavelength of 1064 nm. The highly reflective gratings of the fiber may then be selective to reflect the 1064 nm wavelength, and the pump sources may themselves be optical fibers doped, preferably with ytterbium (Yb), and pumped with optical energy at a wavelength of, for example, 915 nm.
In another embodiment of the invention, the reflectors for providing oscillation of the pump energy through the gain medium are integrated into a portion of the signal pathway, and may be integrated into the amplifier fiber itself. These reflectors, preferably highly reflective Bragg gratings, are wavelength specific, and do not significantly interfere with the optical signal to be amplified. That is, the reflectors maintain oscillation of optical energy at the pumping wavelength through the gain medium, while the optical signal passes through them and through the doped optical fiber. To cause generation of energy at the pump wavelength, a pump source is coupled into the gain medium and causes amplification of optical energy at the pump wavelength within the gain medium. Thus, the output of the pump source is absorbed by the doped optical fiber, and amplifies the pump energy that oscillates between the two reflectors. The oscillating pump energy, in turn, is absorbed by the gain medium and amplifies the optical signal passing through the fiber. In such an embodiment, a ytterbium-doped fiber may be used. The signal wavelength could then be 1090 nm, the pump energy wavelength 1064 nm, and the pump source wavelength 915 nm.
In one variation of the embodiment having reflectors integrated into the signal pathway, the amplifier is a two-pass amplifier. A signal reflector is provided at one end of the doped fiber that reflects optical energy at the wavelength of the optical signal. The optical signal is then coupled through an input port into the other end of the fiber. The optical signal is amplified as it passes through the fiber, which is pumped by oscillating pump energy. Upon reaching the end of the fiber, the optical signal encounters the signal reflector, and is directed back through the optical fiber, where it is further amplified. At the end of the fiber where it initially entered, the amplified optical signal is coupled out through an output port.
The foregoing embodiment may be accomplished by using an optical circulator, which allows unidirectional coupling of an optical signal from one port of the circulator to another. If the optical signal is input to a first port of the circulator, the amplifier may be located in a branch coupled to a second port, which receives the optical signal from the first port. The amplified optical signal, after passing twice through the gain medium, returns to the second port of the circulator, where it is coupled to a third port. In one version of this embodiment, the third port is simply a system output port. However, a second amplifier, identical to that connected to the second port, may be located in a branch coupled to the third port, and a fourth port of the circulator could then serve as the system output port. Additional amplifier branches can also be added in a similar manner up to the maximum port capacity of the circulator.